No, they will definitely NOT last nearly that long. At best, during the hotter conditions of summer, they will last about 10 days. I've seen them go from fairly tight buds to blasted in five or six days during particular brutal hot spells in July or August. They will last about 3 to possibly 4 weeks later in fall, because the nights are a lot colder as a rule, and it acts as natural refrigeration. All of the above is somewhat variable due to weather conditions and cultivar.

If you want to make mums last "as long as possible" put them in a semi-shady spot when the flowers open, and keep them watered. That will buy you a few extra days.

Bottom line is that you should go back to the original plan, getting your mums sometime within a week of your wedding date, and then be sure to check them every day for water because they can dry out very fast.

Good luck and congrats.